Ion-pair dissociation dynamics of O_2 in the range 17.2-17.5 eV studied by XUV laser and velocity map imaging method

摘要

The ion-pair dissociation dynamics of O_2, O_2 + hv → O~+(~4S) + O~-(~2P _(1/2, 3/2)), in the photon energy range 17.20-17.50 eV has been studied using extreme ultraviolet laser and velocity map imaging method. The ion-pair yield spectrum and the fine structure resolved photofragment O~-(~2P_j) velocity map images have been recorded. The branching ratios between the two spin-orbit components O~-(~2P _(3/2)) and O~-(~2P_(1/2)) and the corresponding anisotropy parameters describing their angular distributions have been determined. It is found that the fragments O-(2P 1/2) are all from parallel transitions, while the fragments O ~-(~2P_(3/2)) are from both parallel and perpendicular transitions. The main products for most of the excitation photon energies are O~-(~2P_(1/2)). The dissociation dynamics has been discussed based on the ab initio potential energy curves of the ion-pairs. The major peaks in the ion-pair yield spectrum have been assigned based on the angular distribution of the photofragments. The experimental results suggest that the so-called strong and weak series of Rydberg states converging to O_2+bΣg-4 should have symmetries of Σu-3 and ~3Π_u, respectively. In addition to the Rydberg states converging to O_2+bΣg-4, the Rydberg states converging to O_2+AΠu2 should also play a role in the ion-pair dissociation of O_2.